Adv 2.0 L2: rip legacy arena combat path; boss flow is the only path

Cancels the planned ARENA_BOSS_FLOW soak. The boss flow is shipped
unconditionally with no legacy fallback — boss-flow errors now surface
to the player and abort the round rather than silently falling back to
the old CombatPower path.

Deleted:
- runArenaCombat (combat_bridge.go) — legacy CombatPower-vs-Lethality
  arena combat driver.
- RenderCombatLogArena, renderArenaOutcome (combat_narrative.go) — legacy
  arena renderers. RenderCombatLogArena was a thin alias for
  RenderCombatLog; renderArenaOutcome was already dead.
- renderArenaCombatFinalMessage (combat_bridge.go) — legacy "rewards +
  closer" trailing text.
- arenaWinCloser, arenaLoseCloser (adventure_arena_combat.go) — flavor
  helpers that only fed the deleted renderers. File now just holds
  arenaParticipationXP.
- arenaBossFlowEnabled + the ARENA_BOSS_FLOW env var (adventure_arena.go,
  .env.example). The env gate served only the now-removed fallback.
- sendArenaCombatMessages (adventure_arena.go) — wrapper that switched
  pacing between boss flow and legacy. Replaced with direct
  sendZoneCombatMessages calls at the three call sites.
- TestArenaBossFlowEnabled (bossflow test) and
  TestRenderCombatLogArena_ProducesPhaseMessages (narrative test).

Simplified resolveArenaRound / resolveArenaSurvival / resolveArenaDeath:
the bossNarr-vs-nil branches collapsed since narration is now always
produced by resolveArenaBoss. Equipment degradation and the death-save
reprieve hook are unchanged — both already worked off the boss-flow
CombatResult.

Migration doc (§4) updated to record the cancellation and the
remaining tuning approach (playtest-driven, no flag soak window).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -37,7 +37,6 @@ FEATURE_SHADE=
 FEATURE_TRIVIA=true  # set to "false" to disable trivia
 FEATURE_ESTEEMED=    # set to anything to enable satirical esteemed member posts
 ESTEEMED_ROOM=       # room ID for esteemed member posts (separate from broadcast rooms)
-ARENA_BOSS_FLOW=     # Adv 2.0 L2: set to "1" to route arena rounds through the zone-boss flow (staged combat log + TwinBee mood lines). Empty / "0" / "false" keeps legacy CombatPower path. Removed when L4f lands.
 
 # ---- Games & Economy ----
 GAMES_ROOM=                    # room ID where game commands work (trivia, hangman, blackjack, holdem, wordle, flip)